Chromium electroplating baths have been in widespread commercial use for many years for applying protective and decorative chromium platings to metal substrates. Heretofore, commercial chromium plating electrolytes conventionally employed hexavalent chromium ions derived by dissolving compounds such as chromic acid, for example, into the aqueous electroplating solution. The use of such hexavalent chromium electroplating electrolytes has been characterized as having limited covering power and excessive gassing particularly around apertures in the parts being plated which can result in incomplete coverage. Such prior art hexavalent chromium plating solutions are also characterized as being sensitive to current interruptions resulting in so-called "whitewashing" of the electrodeposit.
In more recent years, chromium electrolytes have been developed containing substantially all of the chromium in the trivalent state providing many advantages over the prior art hexavalent chromium electrolytes including enabling use of current densities ranging over a broad range without producing any burning of the electrodeposit; minimizing or completely eliminating the evolution of mist or noxious odors during the chromium plating process; providing for excellent coverage of the substrate and good throwing power of the electroplating bath; enabling current interruptions during the electroplating cycle without adversely affecting the chromium deposit thereby enabling parts to be withdrawn from the electrolyte, inspected, and thereafter returned to the bath for a continuation of the electroplating cycle; reducing the loss of chromium due to drag-out by virtue of employing lower concentrations of the trivalent chromium ions; and facilitating waste disposal of the chromium in effluents by virtue of simple precipitation of chromium from such aqueous effluents by the addition of alkaline substances to raise the pH to about 8 or above.
A problem associated with the commercial operation of trivalent chromium electrolytes has been the build-up of hexavalent chromium ions in the electrolyte to a level at which interference with efficient electrodeposition of chromium has been encountered as well as a reduction in the efficiency and covering power of the bath. In some instances, the progressive build-up of detrimental hexavalent chromium ions has occurred to the extent that a cessation in electrodeposition of chromium has occurred necessitating a dumping and replacement of the electrolyte.
The present invention is based on a discovery whereby efficient and continuous electrodeposition of commercially satisfactory chromium platings can be attained employing trivalent chromium electrolytes wherein the tendency to progressively build up concentrations of detrimental hexavalent chromium ions is inhibited or substantially eliminated thereby maintaining the efficiency of the operating bath. Additionally, the process of the present invention further provides for improved stability in the pH of the electrolyte during use so that analysis and periodic adjustment of the operating pH is reduced simplifying operation and control of such trivalent chromium electroplating operations.